all: rename Tlv namespace to tlv for consistency

This commit renames the Tlv namespace defined in
tlv.hpp to tlv to be more consistent with tlv::security
and tlv::nfd. A Tlv namespace alias is provided for
backwards compatibility.

Change-Id: I21278d6a077afe7776802c3296997d4c1c44c6c6
diff --git a/src/security/key-chain.cpp b/src/security/key-chain.cpp
index 180d37d..1809079 100644
--- a/src/security/key-chain.cpp
+++ b/src/security/key-chain.cpp
@@ -550,7 +550,7 @@
   DigestSha256 sig;
   data.setSignature(sig);
 
-  Block sigValue(Tlv::SignatureValue,
+  Block sigValue(tlv::SignatureValue,
                  crypto::sha256(data.wireEncode().value(),
                                 data.wireEncode().value_size() -
                                 data.getSignature().getValue().size()));